It is generally known that the constitution of undiluted samples of air expired from the lungs of a subject is tightly linked to the corresponding blood constitution. This is due to the extremely large surface area of normal lung tissue, including the alveoli combined with a network of fine capillary blood vessels, resulting in efficient gas exchange. Measurement of end tidal gas concentrations, i e the value obtained at the end of the respiratory cycle, is thus replacing arterial blood sampling in clinical physiology. Furthermore, breath sampling is now being accepted in many countries as an evidential method for the assessment of blood alcohol concentration in vehicle drivers.
The reliability, speed and ease with which breath sampling can be accomplished is, of course, a central issue. For example, in the case of alcohol interlocks for vehicle drivers, the risk of false output from the analysis should be brought down to an absolute minimum. A large sample volume is normally required in order to obtain high resolution, opposite to the requirement of fast response, which favours a small sample volume. Only a very small fraction of samples will actually result in the positive detection of alcohol, and sober drivers will not accept a cumbersome, and time-consuming testing procedure. A further complication is that such a device should operate with maintained accuracy also at extreme environmental conditions.
Breath sampling is commonly performed with a mouthpiece to ensure that the sample is undiluted. Typically, the mouthpiece consists of a piece of polymer tubing with openings to expose the sensor to the breath sample. Mouthpieces are disposable items for hygienic reasons, and their handling and cost are major limitations to the widespread use of breath analysing equipment in e g alcohol interlock systems.
Recently, it was demonstrated that the simultaneous measurement of carbon dioxide and another substance of interest in the vicinity of a subject may provide a novel possibility of quantitative assessment of the blood concentration of that substance. It has been demonstrated that the ratio of the two externally measured concentrations multiplied by the alveolar concentration of carbon dioxide will provide an approximation of the actual blood concentration of the substance. The variability of alveolar CO2 concentration is limited and predictable to a large extent, and may therefore be estimated with fair accuracy. The new technique may eliminate the need for a mouthpiece, except when very high measurement accuracy is required.
It is further generally known that many substances in the gas phase exhibit distinctive absorption spectra in the infrared wavelength range between approximately 1 and 10 μm. In fact, absorption spectroscopy is a major tool to determine the composition of unknown gas samples. This is due to quantum mechanical transitions between energy levels of molecular vibrations. Carbon dioxide, for example, exhibits a strong absorption peak at a wavelength of approximately 4.26 μm, corresponding to an asymmetric stretch mode of vibration, in which the central carbon atom vibrates in opposition to the two oxygen atoms along the linear axis of the molecule. Ethyl alcohol exhibits distinctive absorption peaks at 3.4 and 9.4 μm, also corresponding to molecular vibratory states. Water vapour, for comparison, exhibits absorption peaks at 2.8 and 6.2 μm. Determination of water vapour corresponds to the measurement of absolute humidity. Relative humidity may be calculated from this if temperature is known. The simultaneous measurement of humidity in breath samples may be of interest, as will be further described below.
Infrared absorption spectroscopy may involve measurement of the transmission of infrared radiation through the sample from a source of radiation and a detector. Typically, a dispersive element is also introduced in the radiation path, whereby radiation at certain wavelength intervals only, are transmitted to the detector. A diffraction grating or an interference filter could serve as a dispersive element. By varying the angle of incidence, it is possible to vary the accepted wavelength interval. In a scanning spectrometer, the wavelength interval is successively scanned, allowing a certain range of wavelength intervals to be analysed, and thereby the detection of multiple absorption peaks corresponding to one or several substances.
When certain substances are being monitored alone, it is customary to use interference filters as dispersive elements, with transmission properties matched to the absorption peaks of those substances. State-of-the-art interference filters with excellent properties can be produced at low cost, and can be integrated with infrared detectors, e g of the thermopile type.
In photoacoustic spectroscopy, a pulsed radiation source with filter matching certain absorption peaks is being used. In the presence of an absorbing substance, heat pulses synchronous with the radiation pulsations may be detected by a sensitive microphone. This solution is attractive for the detection of substances with very low concentration due to favourable noise characteristics. On the other hand, it is more complex and expensive, especially when a plurality of substances are involved.
Transmission measurements are advantageous from the point of view of reliability. A transmission spectrometer may include self-monitoring functions, including all vulnerable elements. It may, for example, monitor the output from the infrared source, enabling compensation for eventual long term drift.
A technological challenge not solved in state-of-the-art infrared spectrometers is to combine high measurement resolution with fast response. Ideally, the system should respond and recover as fast as normal human perception, i e within a few seconds. On the other hand, the demands on high resolution of weakly absorbing substances infer relatively long radiation transmission paths, of the order of several tens of centimeters. As already pointed out, these requirements are in opposition.
Further difficulties are related to the legal aspects related to the collection and analysis of breath samples from human subjects. The demands on reliability and traceability of eventual errors are exceptionally high. Possible attempts to manipulate the analysing process should be detected in order to allow adequate measures.
A difficulty specifically related to the analysis of breath samples, and the simultaneous CO2 measurement, is the fact that the CO2 concentration in breath samples is typically in the percent range, whereas the concentration of other substances is typically several orders of magnitude lower. It is thus necessary to minimise cross sensitivities, i e the interdependencies between the various determinations, and the large difference in concentrations is a complication.
It may be of interest to perform breath sampling outdoors, and at extreme environmental conditions. State-of-the-art infrared spectrometers are mainly used in laboratory-like environments. It is therefore an objective of the present invention to minimise the environmental influences, and to improve the durability to extreme conditions.
A further objective of the present invention is that the system should allow implementation at high production volumes, and at very low fabrication cost. Physically, it should be useable as a handheld stand-alone unit, or as an embedded system, e g in a vehicle.